<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>

    <div class="box">

    </div>
</body>
<style>
    .box {
        height: 0;
        width: 0;
        border-top: 100px solid #bfc;
        border-bottom: 100px solid transparent;
        border-right: 100px solid transparent;
        border-left: 100px solid transparent;

    }
</style>

</html>